1. Field of the invention
The invention concerns a magnetic bearing for active centering of a second body mobile relative to a first body about at least one axis. The second body can be a body mobile in rotation relative to a stator or a body mobile in translation relative to a fixed body.
2. Description of the prior art
Various magnetic bearings for active centering of a body mobile relative to another body about at least one centering axis known in themselves include first and second portions adapted to be fixed to one or to the other of the bodies, separated by a pair of airgaps of variable thickness parallel to the centering axis. These airgaps extend along a line of relative movement between the portions, this line of relative movement being parallel to the ferromagnetic plates. In the case of a rotor mobile relative to a stator, this line of movement is typically annular; on the other hand, in the case of a body mobile in translation relative to a second body, this line of relative movement is substantially linear. The first portion (usually the portion which is fixed to that of the two bodies which is fixed, which explains why it is usually referred to as the stator portion) includes two parallel ferromagnetic plates between which are disposed a core carrying a coil and permanently magnetized means the magnetization direction of which is perpendicular to said plates. The second portion, typically carried by the mobile body, is made of a ferromagnetic material and closes the fluxes generated in the pair of airgaps by the coil and by the permanently magnetized means.
In a manner that is entirely conventional, the magnetic fluxes generated by the coil and by the permanently magnetized means circulate in substantially coplanar magnetic loops in a plane substantially perpendicular to the ferromagnetic plates and to the line of relative movement so that the fluxes combine simply by addition or subtraction. This is why the permanently magnetized means are typically disposed between the core and the coil, on the one hand, and the pair of airgaps, on the other hand.
A bearing design of the above kind has until now proved an entirely satisfactory response to many requirements. Nevertheless, it has the disadvantage of considerable overall size transversely to the airgaps; this constitutes a handicap to obtaining magnetic bearings of very small size. Moreover, the permanently magnetized means are generally in the form of a magnetized bar extending parallel to the line of relative movement, i.e. to the pair of airgaps. This magnetized bar is therefore annular in the case of a rotor mobile relative to a stator; alternatively, this bar is linear in the case of a body mobile in translation relative to a fixed body. In the situation of centering a rotor relative to a stator, the bearing design of the above type therefore requires the production of a magnetized ring having precise dimensions (in particular its diameter), which means that, to avoid the necessity to maintain a large stock of magnets, the annular magnetized bars have to be made as and when required, which increases the manufacturing times of magnetic bearings for rotor/stator combinations and results in high costs.
With regard to the first disadvantage, concerning the overall size transversely to the airgaps, it has already been proposed to make the core and therefore the coil kidney-bean-shape along the magnetized bars. This enables some degree of minimization of the overall radial size of the bearings for rotors. Note, however, that the efficiency of any such coil is not optimal since the section of the core in which the coil generates a flux is necessarily small, unless a very long coil is acceptable with a large number of turns around the core.
Another disadvantage is that the magnetized bars are generally manufactured by sintering. It is well known that as the size and the volume of dense sintered parts increase it quickly becomes difficult to obtain parts of good quality at moderate cost. There is therefore at present a limitation on the size of bearings for rotors that can be made at acceptable cost.
An aim of the invention is to overcome the aforementioned disadvantages. It is therefore directed to enabling production of the magnets without any prior knowledge of the dimensions or even the design of the bearing for which they are intended. It is also directed to enabling the manufacture of large bearings, up to sizes exceeding those that can currently be made. Conversely, it is directed to enabling the manufacture of very small bearings, in particular with regard to the overall size transversely to the airgaps. Finally, it is aimed at improving the efficiency of the coils, i.e. to increasing the ratio between the section of each turn and its length (for the core this corresponds to the largest possible ratio between its section and its perimeter).